1. Technical Field of the Invention
The present invention relates to a turbo compressor, and more particularly to a turbo compressor in which two centrifugal impellers are fixed to one rotating shaft in such an orientation that their back sides face to each other.
2. Description of the Related Art
In a refrigerating machine, there is employed a centrifugal compressor, so-called turbo compressor, for compressing a coolant gas serving as a working fluid so as to bring the compressor into a high temperature and high pressure state.
Meanwhile, in the compressor, if a compression ratio is higher, a discharge temperature of the compressor becomes higher and a volumetric efficiency is lowered. Particularly, if the evaporation temperature becomes lower, the compression ratio becomes higher, and accordingly, there is a case that a compressing operation is divided into two stages, three stages or more stages. The turbo compressor in which the compressing operation is executed by multi stages in this manner is called as a multistage turbo compressor.
In a two-stage turbo compressor, there has been known a structure in which two centrifugal impellers are fixed to one rotating shaft in the same orientation, and also a structure in which two centrifugal impellers are fixed to one rotating shaft in such an orientation that their back sides face to each other. As a prior art thereof, there is a structure disclosed in the following patent document 1, and the structure is shown in FIG. 1.
This turbo compressor 80 is constituted as a one-shaft two-stage compressor in which a first stage compression impeller blade (a centrifugal impeller) 83 is fixed to one end of a motor shaft 82 rotatably provided in a housing 81, and a second stage compression impeller blade 84 is fixed to the other end of the motor shaft 82 mentioned above. A coolant gas is compressed by the first stage compression impeller blade 83, and is introduced into the second stage compression impeller blade 84 through a piping 85.
As mentioned above, in the turbo compressor in which two centrifugal impellers are fixed in such an orientation that their back sides face to each other, since inlets of both the centrifugal impellers face in the opposite directions, it is general that the flow path between both the centrifugal impellers is formed by a piping connecting the centrifugal impellers.
Meanwhile, in the multi-stage turbo compressor, there is a structure in which an intermediate suction flow path for introducing a gas is provided in an intermediate stage or a final stage between a suction port and a discharge port. For example, in a multi-stage turbo compressor for a refrigerator used for a refrigerating cycle having an economizer, the coolant gas from the economizer is mixed with the coolant gas compressed on the low pressure compression stage such that the mixed gas is recompressed on the high pressure compression stage. The turbo compressor mentioned above is disclosed, for example, in the following patent document 2, and a structure thereof will be shown in FIG. 2.
This turbo compressor 90 is a two-stage turbo compressor in which two centrifugal impellers 91 and 92 are fixed to one rotating shaft 93 in the same orientation. In the turbo compressor 90, a ring-shaped entirely circumferential suction chamber 96 is provided in an intermediate stage impeller inlet passage 95 within a casing 94 such that a gas supplied from an intermediate suction port 97 is introduced into the impeller inlet passage 95. A flow guide 98 constituted by a separator plate and a pair of guide plates is arranged in a portion just near the suction port of the chamber 96.
Thereby, the gas is uniformly supplied from the circumferential direction so as to be uniformly mixed with a main stream, by the entirely circumferential suction chamber 96 and the flow guide 98.
In this case, in FIG. 2, reference symbols A and B denote bearings, reference numeral 99 denotes a motor, reference numeral 100 denotes an output shaft of the motor, reference numeral 101 denotes a large gear fixed to the output shaft 100, and reference numeral 102 denotes a small gear fixed to the rotating shaft 93.    Patent document 1: Japanese Laid-Open Patent Publication No. 5-223090    Patent document 2: Japanese Laid-Open Patent Publication No. 2002-327700
In the turbo compressor in which two centrifugal impellers are fixed to one rotating shaft in such an orientation that their back sides face to each other, such as the prior art shown in the patent document 1 mentioned above, it is general that the flow path between both the centrifugal impellers is connected by the piping.
However, in the case that the flow path between both the centrifugal impellers is connected by the piping as mentioned above, the diameter of the piping and the shape of the curvature affect the structure, and therefore, there is a problem that a product is increased in size and weight.
Further, since the piping itself forms an independent part, the number of parts is increased, and an assembling work time is additionally necessary. Accordingly, there is a problem that a cost increase is caused.
Further, in the prior art shown in the patent document 2 mentioned above, the entirely circumferential suction chamber and the flow guide are provided for introducing the coolant gas from the economizer into the flow path between two centrifugal impellers for uniformly mixing it with the main stream. However, since the special structure and parts are necessary, there is a problem that a cost increase is caused.
Further, since the prior art shown in the patent document 2 relates to the turbo compressor in which two centrifugal impellers are fixed to one rotating shaft in the same orientation, it is impossible to apply the structure in which the coolant gas and the main stream are uniformly mixed, to the turbo compressor in which two centrifugal impellers are fixed to one rotating shaft in such an orientation that their back sides face to each other. Accordingly, in the turbo compressor in which two centrifugal impellers are fixed to one rotating shaft in such an orientation that their back sides face to each other, an effective means for uniformly mixing the coolant gas (injection gas) from the economizer with the main stream is necessary.